In the area of rotors and rotor systems eccentricity means deformation of the rotor or the rotor system in the sense of a bend showing itself when the rotor is being rotated at low revolutions (e.g. by means of turning gear), at which dynamic excitation forces (e.g. from rotating unbalance) and flexibility of the rotor or the rotor system are not effective. With steam turbines for instance, the deformation is usually caused by heterogeneous temperature field formed upon contact of the rotor or rotor system with stator parts, during saturation of shaft seals or during soaking a flow path of the turbine by means of steam before switching the machine to operating speed, during cooling the machine after it has been shut down and operating it by means of turning gear before full stand-still, during temporary interruption of permanent rotor or rotor system rotation on the turning gear when soaking or cooling, etc. If the eccentricity of the rotor or the rotor system is not discovered in time, it can cause damages of the machine and, in extreme cases, its complete breakdown. This danger is increased especially within a band of critical speeds (natural frequencies) where up to multiple increase of amplitude of vibrations of the rotor or rotor system occurs, being caused by high amplitude of excitation force from an unbalance (from the point of view of analysis of dynamic features, the bend itself behaves as an unbalance) as well as by resonance amplification.
At present, the eccentricity of the rotor or the rotor system in the case of steam turbines is determined based on measurement of the rotor or the rotor system deviation against the stator part at low speeds of the machine by means of a sensor (the usual operating principle of which are eddy currents) placed furthest from the rotor or the rotor system bearings towards the centre of the flow path. Subsequently, there are two basic methods to be used to assess the eccentricity. In the first of them, the swing or amplitude of oscillation of the sensor signal at the point of measurement is monitored and measured values are compared to allowed limits. In the other method, time behaviour or trend of the sensor signal is monitored, which is further compared to common values. The method requires increased attention of service personnel and its considerable experiences, which is its major disadvantage. However, both methods may lead to considerably distorted results that could be inconsistent with reality, because the eccentricity of the rotor or the rotor system cannot be uniquely determined solely from time series of measured signals, without appropriate reference signal. Even if the reference signal, measured for example at the moment when the rotor/rotor system would be straight was available by any of the methods, it would not be clear from its comparison to the measured signals whether the possible deviation was caused only by the eccentricity of the rotor or the rotor system or by another kind of defect occurring at the point of measurement—for example by ovality, increased roughness of the surface resulting from long-term operation, etc. None of the current methods thus leads to unique determination of the eccentricity of the rotor or the rotor system, and does not allow its exact diagnostics.
The goal of the invention is to propose a method for determining current eccentricity of rotating rotor which would provide sufficiently exact values and, based on such method, designed diagnostics of eccentricity of rotating rotor.